Custom replacement device for resurfacing a femur and method of making the same

ABSTRACT

A replacement device for resurfacing a joint surface of a femur and a method of making and installing such a 5 device is provided. The custom replacement device is designed to substantially fit the trochlear groove surface, of an individual femur. Thereby creating a “customized” replacement device for that individual femur and maintaining the original kinematics of the joint. The replacement device may be defined by four boundary points, and a first and a second surface. The first of four points is 3 to 5 mm from the point of attachment of the anterior cruciate ligament to the femur. The second point is near the bottom edge of the end of the natural articulatar cartilage. The third point is at the top ridge of the right condyle and the fourth point at the top ridge of the left condyle of the femur. The top surface is designed so as to maintain centrally directed tracking of the patella perpendicular to the plane established by the distal end of the femoral condyles and aligned with the center of the femoral head.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.09/528,128, filed Mar. 17, 2000 now U.S. Pat. No. 6,712,856, hereinincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a replacement device for a kneejoint, and more particularly, to a device for resurfacing the trochleargroove of a femur that is customized to an individual and to methods forinstalling and making the same

2. Description of the Related Art

The human knee joint primarily includes three parts, anatomicallyreferred to as the femur (thighbone), the tibia (shinbone) and thepatella (kneecap). The knee joint is then further subdivided into twojoints: the patella-femoral joint (space between the kneecap and distalanterior surface of the thighbone) and the tibia-femoral (space betweenthe thighbone and shinbone).

During normal bending and straightening of the leg, the patella(kneecap) slides over the femur (thighbone) within a groove that islocated on the front distal surface of the femur. This groove isreferred to as the trochlear groove. Several types of abnormalities canoccur with the movement of the patella over the femur. For example, thepatella may dislocate or slip out of place, it may fracture, or developa tracking problem. Normally, the patella tracks, or glides within thecentral region of the trochlear groove. A tracking problem occurs whenthe patella no longer remains centered within the groove as it glidesover the femur. The resulting abnormal biomechanics can cause chronicpain in the joint and if left untreated, it can lead to degenerativearthritis.

The distal end of the femur (within which resides the trochlear groove)is covered with articular cartilage. This cartilage functions as acushion between the femur and the tibia. In arthritis of the knee joint,the articular cartilage breaks down, either from abnormal wear asmentioned above, or from injury, age, congenital predisposition,inflammatory arthritis, or obesity, etc. When this cartilage breaksdown, the cushion is lost, resulting in pain, swelling, bone spurformation and/or decreased range of motion of the knee joint.

Due to the inability of damaged cartilage to repair itself after injury,the range of treatment for patients with unicompartmental diseaseinvolving the patella is limited. The most commonly prescribedtreatments included soft tissue releases and/or realignment of thepatellar tendon, patellectomy, where the patella is completely removed,or a total knee replacement with a standardized patello-femoral-tibialprosthesis. In certain instances none of these procedures may bedesirable or effective. For example, the soft tissue procedures may notwork. A patient having undergone a patellectomy is left partiallycrippled due to the loss of the kneecap, which served to hold the jointtogether. Additionally, these patients often still suffer from pain dueto contact of the remaining tendon moving directly over the groove. Atotal knee replacement with a standardized prosthesis is also far fromideal because much of the femur bone must be carved away in order to“fit” the distal surface of the femur to the standardized prosthesis.Additionally, the patients are often young and are likely to requirereplacement of the prosthesis. Each revision operation is moredifficult. Therefore, there still is a need for a better treatment ofpatient's with degenerative arthritis of their patella-femoral joint.

The present invention provides a replacement device that is customizedto each individual knee joint. Due to the “fitting” of the replacementdevice to the patient's femur, instead of the reverse as is the casewith standard prosthetic devices, the patient's original range and forceof motion (kinematics) through the knee joint is preserved and thepatient does not suffer from device-related pain. Also included is amethod for making a customized replacement device and a marking templatefor such a device. Bone stock removal is limited and functionally ismaximized.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed toward a custom replacement device forresurfacing an articulating or joint surface of the femur and methods ofmaking and installing such a device. This custom replacement deviceovercomes the problems associated with prior knee joint replacementdevices or prostheses in that it is made specifically to fit thetrochlear groove surface (surface over which the patella slides) of afemur from an individual patient. Thereby creating a “customized”replacement device for that individual femur.

The replacement device is substantially defined by four outer points andfirst and second surface areas. The first of four points is definedapproximately as being 3 to 5 mm from the point of attachment of theanterior cruciate ligament to the femur. The second point is definedapproximately at or near the superior edge of the end of the naturalcartilage. The third point is defined approximately at the top ridge ofthe right condyle. The fourth point is defined approximately at the topridge of the left condyle. The first surface area is customized tosubstantially match the bone surface area of the trochlear groove of thefemur. The second surface area has a tracking path that is approximatelyperpendicular to the end of the condyles of the femur. The thicknessbetween the first and second surface areas may be approximately between2 mm and 6 mm. To couple the replacement device to the femur, a pinprotruding from the first surface area may be used to penetrate anopening in the femur prepared by a surgeon. Boney ingrowth may securethe prosthesis or bone cement may be used.

The replacement device can also include a customized drill guide that issubstantially defined by first and second surface areas. The firstsurface area is customized to the surface area of the trochlear grooveof the femur. The second surface area includes a hole that is alignedsubstantially to the pin to assist in drilling the opening into thefemur for the pin.

In accordance with one aspect of the present invention, these and otherobjectives are accomplished by providing a replacement device having atop surface; a bottom surface; the bottom surface substantially formedto match the trochlear groove surface of a femur; and the top surfacesubstantially tracking the trochlear groove of the femur.

In accordance with another aspect of the present invention, theseobjectives are accomplished by providing a system for installing areplacement device to a distal end of a femur having a trochlear groovesurface, comprising: a marking template, wherein: the marking templatehas a back side substantially matching the distal end of a femur; and anopening through the marking template; a drilling apparatus to form ahole on the distal end of the femur assisted by the opening in themarking template; and a replacement device, wherein: the replacementdevice has a bottom side substantially matching the distal end of thefemur; and a pin protruding from the bottom side of the replacementdevice adapted to insert into the hole on the distal end of the femur.

In accordance with yet another aspect of the present invention, theseobjectives are accomplished by providing a method of making areplacement device, comprising the steps of: forming a model of a distalend of a patient's femur; forming a first mold from the model, whereinthe first mold has a bottom side that substantially matches thetrochlear groove of the patient's femur, wherein the first mold has atop side opposite of the bottom side; coupling a peg on a predeterminedlocation on the bottom side of the first mold; shaping the top side ofthe mold to substantially track the trochlear groove of the patient'sfemur; forming a second mold from the first mold; and pouring viscousmaterial into the second mold to make a replacement device.

In accordance with still another aspect of the present invention, theseobjectives are accomplished by providing a replacement device having abottom side that substantially matches the trochlear groove of apatient's femur, wherein the bottom side of the replacement device has apin at a predetermined location; providing a marking template having aback side that substantially matches the trochlear groove of thepatient's femur, wherein the marking template has an openingcorresponding to the predetermined location of the pin; removing thecartilage from the distal end of the femur; positioning the markingtemplate about the femur substantially similar to the desired installedposition of the replacement device; drilling a hole on the distal end ofthe femur though the opening of the marking template; removing themarking template from the femur; and inserting the pin of thereplacement device into the hole of the femur to install the replacementdevice on the desired location of the femur.

Alternatively, a method of forming a customized replacement device for afemur will include the steps of duplicating the surface of the distalanterior femur from an individual; and using the duplicate to form aback surface of the customized replacement device and/or a customizedmarking template.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWING

FIG. 1 is a perspective view of; a model femur showing the patellar sidewith its trochlear groove, and the replacement device adapted to coupleto the femur;

FIG. 2 is a perspective view of one embodiment of the present inventionillustrating an exemplary replacement device coupled to the distal endof a femur;

FIG. 3 is an another perspective view of the femur with the replacementdevice coupled to the femur;

FIG. 4 is yet another perspective view of the femur associating with acustom marking template having guide holes that correspond to the pin ona replacement device, residing on the trochlear groove surface of thefemur;

FIG. 5 is a cross-sectional view of the replacement device residing onthe femur, along the line 5—5 in FIG. 2;

FIG. 6 is the perspective view of FIG. 4 with a patella residing on thetrochlear groove surface of the replacement device;

FIG. 7 is an exemplary illustration of a human femur attached to a tibiaby the anterior and the posterior cruciate ligaments;

FIG. 8 is an expanded view of a human knee joint in a bent position;

FIG. 9 is a perspective view showing a patellar face side of a humanfemur and a back side view of a replacement device prior to placement ofthe replacement device onto the femur;

FIG. 10 is a diagrammatic chart of a method of making a mold forconstructing a replacement device; and

FIG. 11 is a diagrammatic chart of a method of making a replacementdevice and a marking template from a femoral mold.

DETAILED DESCRIPTION OF THE INVENTION

This description is not to be taken in a limiting sense, but is mademerely for the purpose of illustrating the general principles of theinvention. The section titles and overall organization of the presentdetailed description are for the purpose of convenience only and are notintended to limit the present invention.

One of the features of the present invention is to provide a replacementdevice for a patient's knee joint that replicates as closely as possiblethe original kinematics of the patient's knee. More particularly, thereplacement device substantially replicates patient's actual trochleartracking pattern of the femur to maintain the original articulatingmovement of the knee. To do so, unhealthy articular cartilage is removedand replaced with the replacement device, which is custom fitted for apatient's femur to maintain as closely as possible the originalarticulating movement of the patella about the trochlear groove. Thatis, the replacement device is custom fitted so that the underside of thepatella articulates about the femur approximately two (2) to six (6) mmaway, which mimics what a patella on a healthy articular cartilage wouldarticulate from. The above feature may be accomplished by providing atrochlear groove that is formed along the replacement device that tractsthe trochlear groove on the femur. Moreover, as further discussed below,the underside of the replacement device substantially matches the faceof the femur, to minimize any error in positioning the replacementdevice about the femur. With this introductory in mind, more detaileddescription of the invention is discussed below.

As illustrated by way of example in FIGS. 2 and 9, one embodiment of thepresent invention includes a custom replacement device 4 adapted toassociate with the distal end of a patient's femur 2. FIG. 2 shows amodel made of the patient's femur in FIG. 9. The model 200 issubstantially similar to the patient's femur 2 and is used to make thecustom replacement device 4 (as discussed below). The surface near thedistal end of the femur 2 defines a patellar face 5, and along thepatellar face is a trochlear groove 3 of the femur. FIGS. 7 and 8 showexemplary views of a normal, intact knee joint. Referring to FIG. 8, ona healthy knee, the trochlear groove would be covered with about 5 mm ofarticular cartilage 100. However, if the articular cartilage wears downfor any reason, the cushion and sliding surface that the cartilageprovides is lost, resulting in pain, and therefore may need to bereplaced with the custom replacement device 4.

One of the advantages with the replacement device 4 is that it is customfitted for a patient's femur so that the replacement device 4 replacingthe articular cartilage will maintain as closely as possible theoriginal kinematics of the patient's knee. That is, the replacementdevice 4 is customized specifically to fit the patellar face 5 surfaceand trochlear groove 3 region of the femur 2. In this regard, asillustrated in FIGS. 1 and 9, the back surface 6 of the replacementdevice 4 substantially matches the surface area of the patellar face 5of the femur 2. Moreover, the back (bottom) surface 6 also matches andfits to the contours of the trochlear groove region 3 of the patellarsurface. The custom replacement device is coupled to the femur throughone or more hole(s) 202 in the distal end of a human femur. The hole(s)202 can be made by a surgeon and may be located so as to reside alongthe trochlear groove region 3 of the human femur 2. As shown in FIG. 5,the replacement device can be implanted into the femur via a pin 19inserted into the one or more corresponding hole 202, thus resurfacingthe patellar face surface of the human femur. The front (top) surface 7of the replacement device is contoured to maintain the tracking of thepatella centrally within the front surface during articulation of thepatella about the trochlear groove (FIG. 2).

As illustrated by way of example in FIGS. 1 and 2, the replacementdevice 4 may 10 be substantially defined by four boundary conditions 8,10, 12, 14, along with the back surface 6 and the front surface 7. Byway of background, a human knee joint includes two cruciate ligamentswhich are located in the center of the knee joint. As shown in FIG. 8,these two ligaments, referred to in the art as the anterior cruciateligament (ACL) 22 and the posterior cruciate ligament (PCL) 24, are theprimary stabilizing ligaments of the knee. The ACL 22 attaches, at itsbottom 26, to the tibia 30, and, passes obliquely upward to insert 20into the inner and back part 32 of the outer condyle 15 of the femur 2.Attachment of the ACL to the femur stabilizes the knee joint along therotational axis and prevents the femur from sliding backwards on thetibia (or the tibia from sliding forward on the femur).

In one embodiment of the present invention, the distal portion of thepatellar tongue or first boundary condition of the replacement devicemay be designed to not extend far back to impinge upon the tibialsurface or cartilage of the tibia. Therefore, the first boundarycondition of a replacement device may be defined to be approximately 3to 5 mm from the point of attachment of the ACL to the patient's femur.Specifically, as illustrated in FIGS. 3 and 8, the first boundarycondition 8 may be approximately 3 to 5 mm from the point of attachment20 of the anterior cruciate ligament 22 to the femur 2. In other words,there is approximately 3 to 5 mm distance between the patellar tongueregion (first boundary condition) 8 and the attachment 20 of theanterior cruciate ligament 22. Alternatively, the first boundarycondition of a replacement device 8 is of a sufficient distance abovethe groove 21 where the ACL exits so as not to result in impingementupon the tibial surface 33 during functioning.

Referring to FIGS. 1 and 8, the second boundary condition 10 may bedefined approximately at or near the superior edge 100 of the end of thenatural cartilage 102. As shown in FIGS. 3 and 8, the third boundarycondition 12 may be approximately at the top ridge of the right condyle104. The fourth boundary condition 14 may be approximately at the topridge of the left condyle 106. The four boundary conditions 8, 10, 12,14 and front 6 and back 7 surface, as described above, may substantiallydefine the perimeter or outer edges of the replacement device inapproximately an oval shape. However, the replacement device need not bedefined by any one of the four boundary conditions. Alternatively, thereplacement device may be defined by any one of or any combination ofthe four boundary conditions. Of course, the perimeter or outer edges ofindividual replacement devices may differ depending on the uniqueanatomic characteristics of an individual patient's femur. Besides thefour boundary conditions, the back surface 6 is customized to match thepatellar face 5 of the femur 2 (FIG. 1). Moreover, edges of thereplacement device may be streamlined to have smooth or rounded edges sothat the replacement device may come in direct contact with the bonysurface of the femur without etching away at the femur. Still further,the streamlined edges prevents tearing or damaging the soft tissuearound the knee.

As further illustrated in FIGS. 2 and 3, the front surface 7 isgenerally concave, which is formed by the inner and outer lateral (side)lip regions being raised to contour around the third and fourth boundaryconditions 12 and 14, respectively. Accordingly, as illustrated by wayof example in FIG. 3, the longitudinal path of the resulting frontsurface substantially replicates the actual trochlear groove-trackingpattern for a healthy knee generally in two ways. The first way is tocreate a mold that substantially replicates the distal end of apatient's femur (as discussed below), and based on the geometry of thereplicated mold, the patient's trochlear groove tracking pattern can bedetermined off of the mold. As illustrated by way of example in FIG. 3,the second way is to align the tracking pattern (axis t—t) along thefront surface area 7 so that it is approximately perpendicular to theends of the condyles 104 & 106 of the femur 2, i.e., and aligned withthe center of the femoral head (axis f—f); as most patients have atracking pattern that is generally perpendicular to the ends of thecondyles. Thus, the first method can be used to check the alignment oftracking pattern calculated by the second method, and vice versa.Alternatively, the tracking pattern produced by both methods can becombined to produce an average tracking pattern of both methods. Eitherway, the tracking pattern produced by both methods will substantiallyreplicate the correct trochlear groove-tracking pattern. Without suchcustomization of the replacement device, there are too many variationsamongst patients' knees such that the original kinematics for a patientcould not be reproduced. In addition, standard devices require theremoval of large amounts of bone in order to make them fit onto thefemur. Still further, other known or new methods of tracking thepatient's trochlear groove of the femur may be used.

As illustrated by way of example in FIG. 6, the tracking pattern isgenerally formed along the base of concave surface 313 of thereplacement device 4, thereby maintaining the patella 30 centrallywithin the device 312. Therefore, regardless of the direction or angleof movement of the patella and tibia (shin bone) along the replacementdevice, the trajectory of the patella will be maintained centrallywithin the intercondyle space and the patella will glide smoothly withinthe central line due to the increased curvature of the device along itslateral and medial or side lip regions. This allows the presentinvention to use a standard dome patellar prosthesis 30 such as theKinamed, Inc. GEM Knee System patella, which has a large contact area 34with the device, to distribute the stress applied to the patella and thedevice. So that even if there is a slight misalignment with replicatingthe trochlear groove of the femur, if at all, there is room for errorand still full contact across the surface of the patella to distributethe load in an optimal manner. Accordingly, the patella will repeatedlyglide over the front surface of the replacement device, reliably, andthere is little chance, if at all, that the patella will dislocate fromthe tracking surface of the replacement device. In addition, the largecontact area distributes the load and minimizes peak contact stress inthe polyethylene thus reducing wear debris generation.

In the past, replacement of a diseased knee joint required surgicalmodification of the surface of the femur so as to allow a close “fit” ofthe prosthetic device within the new joint. This required extensiveremoval or carving of the cartilage and bone surfaces of the head of thefemur in order to “match” with the back surface of a standardizedprosthetic device. With the present invention, there is almost noremoval of original bone and therefore, no loss of requisite anatomicalstructures. Insertion of the replacement device requires minimal removalof existing anatomic structures, if at all. Therefore, the intent is toremove only the diseased portion (the natural cartilage) of thepatient's knee joint prior to installing the replacement device.

In a healthy knee joint, the average thickness of the articularcartilage 102 at the knee joint is about 4 to 5 mm (FIG. 8). In otherwords, there is about 4 to 5 mm of articular cartilage covering thefemur, so that the patella articulates about 4 to 5 mm from thetrochlear grove of the femur. In the case of unhealthy cartilage, wherethe replacement device is needed, the unhealthy cartilage is removed andreplaced with the replacement device, which has been custom made for thepatient's femur. To replicate as closely as possible the originalkinematics of the patient's patello-femoral joint, as illustrated by wayof example in FIG. 2 (and further detailed in FIG. 5), the thicknessbetween the back 6 and front 7 surface areas of the replacement devicemay vary approximately between 2 mm and 6 mm. In other words, thethickness of the custom replacement device maybe approximately that ofthe patient's original articular cartilage. To further enhance theinteraction between the custom replacement device 4 and the femur 2, apin 19 may be located at a predetermined position along the back surface6 of the replacement device. One or more pin(s) 19 can protrude from theback surface area to enhance the association of the replacement devicewith the femur. As illustrated in FIG. 9, the replacement device mayhave three pins that insert into the corresponding holes in the femur,so that there will be very little play, if any, once the pins areinserted into the holes. Alternatively, other apparatus and methods maybe used to couple the replacement device to the femur, for example, abone ingrowth surface or adhesives, including cement, may be appliedbetween the replacement device and the femur. Still another alternativemay be a screw or bolt, where the screw penetrates through the openingin the replacement device and into the femur. In other words, any knownor new apparatus or methods may be used to couple the replacement deviceto the femur.

By way of background, a bone ingrowth surface is a surface treatmentapplied to an implant which is designed to allow the patient's bone toattach or ingrow into the prosthesis, and thereby securing theprosthesis to the bone. The surface coating can assume a number ofdifferent forms. For example, commonly used is a process where a layeris physically attached to the prosthesis through titanium plasmaspraying. Other techniques involve sintering small beads or mesh to thesurface or applying a layer of hydroxyapatite may also be used.

With regard to surgically implanting the replacement device, asillustrated in FIG. 4, a customized marking template 300 may be providedthat is substantially defined by first (back or bottom) and second(front or top) surface areas 302 and 304, respectively. Like the backsurface 6 of the replacement device 4, the first surface area 302 of themarking template 300 is customized to match the surface area of thetrochlear groove 5 of the femur so that the marking template will bepositioned properly with the femur. The second surface area 304 includesa hole 306 or holes (depending upon the number of pins protruding fromthe replacement device) that is in a predetermined position to alignsubstantially to the pin on the customized replacement device.Furthermore, the holes or slots 306 of the second surface area 304 ofthe custom marking template, extend through to the first surface 302 ofthe custom marking template. Therefore, each hole or holes 306 in thepredetermined positions of the marking template serve as guides forformation of openings 202 in the femur 2 of the patient into which thepin(s) 19 will be inserted. Moreover, the custom marking template 300may be substantially defined by four boundary conditions as in thereplacement device 4. Accordingly, the custom marking template 300 asshown in FIG. 4, assists the surgeon in identifying the perimeter of thecustom replacement device 4 as well as the location of opening 202(depending upon the number of pins protruding from the back surface area6 of the custom replacement device 4) that will be drilled into thepatellar face surface of the femur 2.

To surgically implant the replacement device to the patellar face 5 ofthe femur 2, a surgeon may first need to remove some or all remainingdiseased or damaged articular cartilage 102 on the patellar surface 5 ofthe femur (FIG. 8). The surgeon may then scrape away the articulatecartilage until a substantial bony surface 37 of the patellar faceshows. FIG. 8 shows a view of a patient's femur prior to preparation bythe surgeon for insertion of a replacement device. Thereafter, themarking template 300 is aligned and positioned onto the patellar face todrill the necessary openings for the pins (FIG. 4). Since the firstsurface area 302 of the marking template matches the contours of thepatellar face 5 along with the four the boundary condition 8, 10, 12 and14, the surgeon is assured that the marking template is aligned andpositioned properly. In other words, the custom marking template can beused to guide the surgeon in marking the location of the openings andthereby aid in the formation of the openings at their appropriatelocation or predetermined positions. Once the marking template is inposition, the surgeon can precisely drill the openings, aided by theholes or slots on the marking template, using any drilling method knownto one of ordinary skill in the art. FIG. 9 shows a representative femur2 where the diseased natural cartilage has been removed, thus exposingthe bony surface of the patellar face 5 of the femur and the openings202 into which the pin(s) 19 of the replacement device 4 will insert.

With the necessary openings 202 precisely drilled into the patellar face5 of the 10 femur, the marking template is removed and replaced with thereplacement device 4. As illustrated in FIG. 9, the custom replacementdevice is placed onto the femur by insertion of the pin 19 or pinsprotruding from the back surface of the custom replacement device, intothe corresponding openings in the predetermined location formed in thepatellar face surface of the femur. The custom replacement device can beaffixed to the bone 20 by a surgical cement, such as Howmedica Simplex PCement, or any other method known to one ordinarily skilled in the art.Once in place, the replacement device provides the patient substantiallyfull motion originally provided by the native anatomical knee joint. Inother words, in most, if not all, positions of the leg, and throughoutthe normal range of movement of the leg, the custom replacement devicepermits normal rotation and lateral movement substantially similar tothe original kinematics of the healthy knee joint. Therefore, thereplacement device 4 gives the patient the “most natural” replacement ofthe knee possible. Note that with the present invention, there is noneed to carve away at the femur, since the backside of the replacementdevice matches the surface of the femur.

FIGS. 10-11, illustrate by way of example an exemplary method ofproducing the replacement device 4 and the marking template. Initially,a three-dimensional model of the distal end of the femur is formed. Thismay be accomplished in step 62, where the distal end of the femur is CTscanned to form a number of slices that are predetermined distancesapart along the longitudinal axis of the femur, such that when theslices are assembled they form a three-dimensional contour of the femur.Furthermore, to ensure that the slices are aligned properly when theyare assembled, a reference point, such as X and Y coordinate may bemarked on each of the slices as a reference point. For example, thescanning may be done through a X-ray or CT scan protocol that defines anumber of slices to be taken, and the starting and stopping marks todetermine where to start and stop scanning. Moreover, in critical areasas in the trochlear groove of the femur where accuracy is particularlyimportant, the distance between the slices may be reduced to moreaccurately define the contour of the femur in that area. In this regard,the slices may generally be about 3 mm apart, but for more accuracy theslices may be about 1.5 mm apart. Of course, the distance between theslices may further vary depending on the accuracy desired. In otherwords, in such areas where more detail regarding contour is desired,like the four boundary regions 8, 10, 12 and 14, and the trochleargroove 5 region, the distance between the slices may be reduced to about1.5 mm or less.

To further improve the accuracy of the sliced images of the femur, anX-ray motion detection rod may also be used to detect movement by thepatient while the CT scan is taken. That is, while the CT scan is taken,any movement by the patient can distort the accuracy of sliced images.To correct for this distortion, a detection rod may be used to detectthe movement of the patient, by moving with the patient, so that whenthe sliced images are later analyzed, the movement by the patient can bedetected and the sliced images can be adjusted for the movement. Stillfurther, a scaling rod may also be used to ensure that the sliced imagesare at a proper scale so that when the sliced images are assembled, thesize of the femur model is correct.

As illustrated in step 64, once the sliced images are taken of thefemur, each sliced image is transferred onto a film, and reviewed for anumber of features, such as clarity, scaling, and whether the imageswere taken according to the protocol. If the images were properly taken,then in step 66, each of the film is cut around the perimeter of thesliced image of the femur. Each cut film may also be marked so thatrelative position of each of the cut film can later be determined. Instep 68, from all of the cut films, a respective small plate may be madefrom each of the cut films having the image of the cut film and themarked information. In step 70, these plates are then used to digitizethe image into a computer.

In step 72, with digitized information of each individual slices, andthe spacing 30 between the sliced images, a computer aided design (CAD)system with a macro may be used to lay all of the sliced images on to aplanar format. In the planar format, knowing the relative position ofeach sliced images relative to one an another, reference holes are drawnto each of the images, so that when the slices are assembled about thereference holes, an accurate model of the femur may be reproduced. Thedigitized sliced images are assembled about the reference holes to viewthe computer generated image of the femur.

In step 78, to replicate a femur from the digitized information, aComputer Aided Manufacturing (CAM) file may be used to cut plasticplates of the digitized information. For example, a laser or any othermethod know to one of ordinary skill in the art may be used to cut theplastic plates representing the perimeter of the sliced images of thefemur, along with the reference holes. Accordingly, individual plasticplates representing a respective sliced image of the femur is produced.In step 80, these plates may be checked for accuracy and quality againstthe films produced earlier. Of course, if the cut plate does notcorrespond with the respective film then another plate may be cut tomatch the image and scale of the film.

In step 82, with the accurate plates, they are assembled in theirrespective positions and spaced apart according to the distance in whichthe sliced images were taken. That is, plates are orderly inserted intoa rod through the reference holes so that the plates align properly.Furthermore, as stated above, in areas where accuracy is important, theslices may be about 1.5 mm apart, while other slices may be about 3 mmapart. The distance between the plates is maintained by a spacer betweenthe plates, where the thickness of the spacer will also vary dependingupon the desired accuracy. The thickness of the plates, themselves, mayalso vary depending on the desired accuracy. In other words, in thetrochlear groove area the plates themselves may be thinner and spacedcloser together to other plates. With regard to the spacers, they may beformed from the reference hole pieces cut from the plates.

In step 84, with the plates assembled, using the edges of the plates asthe outer boundary, the spaces between the plates and the incrementaldifferences in the outer boundary of the plates are covered with filler,such as clay, to form an outer surface of the femur (also shown in FIG.1). In other words, just enough filler is used to cover the edges of theplates to smooth out the edges between the plates to define the outerface of the femur. This forms a model of the distal end of the femur.Although one method of forming a model is discussed here, other known ornew methods of forming a model of the femur may also be used. Forexample, the CT image data may be compiled in a computer, a surfacecreated, and the model machined directly using the surface data to drivea computer assisted machining system.

In step 86, from the model, a mold is made. This can be done for exampleby placing the model into a standardized box and pouring liquid rubberinto it to make a mold. Referring to FIG. 11, in step 92, with the mold,a second model of the femur may be made by pouring, such liquid asurethane into the mold. This step may be repeated to make three modelsof the femur. Two of the models may be sent to the physician to reviewand check for accuracy and affirm the exact location of the boundarypoints.

In step 94, with the second model of the femur, the boundaries aremarked where the replacement device is to be placed on the femur. Thenthe edges of the second model are squared off such that it is outsidethe marked boundaries where the replacement device is to be located.Additionally, the four boundary conditions that may substantially definethe replacement device, along with the outer perimeter of the device maybe marked using transfer ink on the second model. Thereafter, in step96, a third mold may be made using machinable material such as plasticglass, from the squared off second model so that the underside of thethird mold contours the trochlear groove of the femur and marked withthe transfer ink indicating the perimeter of the replacement device. Instep 97, on the underside of this third mold is marked for pegs.

The third mold is then machined or sanded along the top surface togenerally give it a uniform surface throughout. For example, the topsurface of the third mold may be sanded until a uniform thickness of 5mm is relatively formed or to a desired thickness between the top andbottom surfaces of the replacement device. Additionally, the edges ofthe third mold are also machined to give it desired finish. Accordingly,a mold for the replacement device may be made of plastic glass, forexample, generally having an oval shaped defined by the four boundaryconditions and the concave upper surface. To improve the accuracy of themold, the third mold may be made slightly larger than the actual size ofthe replacement device, as cast mold has tendency to shrink. As anexample, the mold may be made about 2% larger than the actual size ofthe replacement device.

In step 101, to form a pin that protrudes from the underside of thereplacement device, a hole for each of the pin is drilled through thethird mold, and inserted with a peg representing the pin. As an example,underside of the replacement device may have three pins placed inequilateral triangle format to distribute the load evenly. Furthermore,the pins may be distributed in predetermined distance apart tostandardize the drill pattern for all of the replacement device.Additionally, the equilateral triangular pins may have its center pointabout halfway between the right and left condyle extents, where the pinsgenerally protrude perpendicularly from the underside of the replacementdevice.

In step 103, the metal replacement device and the marking template aremade from the third mold having the pegs protruding from it. In step105, to produce a replacement device, a fourth female mold is made fromthe third mold, using such material as rubber. From the female mold,liquid metal can be poured into it, to produce a metal replacementdevice. In step 107, to produce a marking template, the pegs on thethird mold is removed and a fifth female mold is made, using suchmaterial as rubber. Then from the fifth female mold without the pegs,liquid metal is poured into it, to produce the marking template.Alternatively, any other method known to one of ordinarily skilled inthe art of producing the replacement device and the marking templatefrom the third mold is within the scope of the present invention. It isalso possible to machine the replacement device directly by creating acomputer model of the device and using said model to drive a machinetool to cut the model directly from a piece of material.

With regard to material, the replacement device 4 is made of asubstantially firm, non-bioreactive material. Examples of such materialinclude, surgical grade Co Cr alloy or other substantially rigid,bio-compatible material such as titanium, stainless steel, zirconia, oralumina ceramic. All surfaces of the device, which are external to thefemoral bone (once the device is implanted), are highly polished andsmoothed.

In closing, it is noted that specific illustrative embodiments of theinvention have been disclosed hereinabove. Accordingly, the invention isnot limited to the precise embodiments described in detail hereinabove.With respect to the claims, it is applicant's intention that the claimsnot be interpreted in accordance with the sixth paragraph of 35 U.S.C. §112 unless the term “means” is used followed by a functional statement.

1. A method of making a replacement device, comprising the steps of:forming a model of a distal end of a patient's femur; forming a firstmold from the model, wherein the first mold has a bottom side thatsubstantially matches the trochlear groove of the patient's femur,wherein the first mold has a top side opposite of the bottom side;coupling a peg on a predetermined location on the bottom side of thefirst mold; shaping the top side of the mold to substantially track thetrochlear groove of the patient's femur; forming a second mold from thefirst mold; and pouring viscous material into the second mold to make areplacement device.
 2. A method according to claim 1, further comprisingthe steps of: streamlining the edges of the replacement device.
 3. Amethod according to claim 1, further comprising the steps of: shapingthe replacement device to have an oval shape defined by first, second,third, and fourth boundary conditions, wherein: the first boundarycondition being approximately 3 mm to 5 mm from the attachment of ananterior cruciate ligament to the femur; the second boundary conditionbeing approximately at least near the superior edge of an end of anatural cartilage of the femur; the third boundary condition beingapproximately at the top ridge of a right condyle of the femur; and thefourth boundary condition being approximately at the top ridge of a leftcondyle of the femur.
 4. A method according claim 1, further comprisingthe steps of: shaping the top surface of the replacement device to havea substantially similar thickness between the top and bottom surfaces,wherein the thickness is approximately between 2 mm and 6 mm.
 5. Amethod according to claim 1, further including the steps of: taking apredetermined number of sliced images along the distal end of apatient's femur transposing each of the predetermined number of slicedimages into a plate; cutting the sliced images from each of the plates;assembling each of the plates to define outer edges of the distal end ofthe femur; and applying filler over the outer edges to form the model ofthe distal end of the femur.
 6. A method of forming a replacement deviceand a marking template device from a single mold, comprising the stepsof: forming a model of patient's distal end of a femur, forming a firstmold from die model, wherein the first mold has a back side that matchesthe trochlear groove of the femur, wherein the first mold has a faceside opposite of the back side; shaping the face side of the first moldto substantially track the trochlear groove of the femur; forming asecond mold from the first mold; and pouring a first viscous materialinto the second mold to make a replacement device.
 7. A method accordingto claim 6, further including the stops of: coupling a peg to the backside of the first mold at a predetermined location; removing the pegfrom the back side of the first mold; forming a third mold from thefirst mold without the peg on the back side; and pouring a secondviscous material into the third mold to make a marking template.
 8. Amethod according to claim 6, further including the steps of: forming anopening through the first mold along the predetermined location.
 9. Amethod according to claim 6, wherein the first viscous material andsecond viscous material is the same material.
 10. A method accordingclaim 6, wherein the step for forming the model of patients distal endof the femur further includes the steps of: compiling in a computer a CTimage data of the patient's distal end of the femur; creating a surfaceof the patients distal end of the femur; and driving a computer assistedmachine system to machine the model of patient's distal end of thefemur.